Discriminated unions are an authoring signal

Designing the notes schema, I kept reaching for “make every field optional and add a refine() for the rules.” Then I remembered the marginalia schema already uses a discriminated union, and switched.

The shift wasn’t about types — it was about authoring. With a flat optional schema, the editor lets you type kind: link and forget the URL. With a union, the build fails the moment you do that. The schema becomes a checklist you can’t ignore. That’s the actual win — not the type narrowing in the renderer, but the constraint at write time.